net: ethernet: renesas: rcar_gen4_ptp: Prepare for shared register layout

All known R-Car Gen4 SoC share the same register layout, rename the
R-Car S4 specific identifiers so they can be shared with the upcoming
R-Car V4H support.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>